The present invention relates to dual waveband optical systems, and more particularly to anti-reflecting coatings with simultaneous utility in both the millimeter-wave and infrared bands.
Anti-reflective coatings utilize the principle of destructive interference to reduce the reflectance from a surface. Differences in the index of refraction of a first and a second media, for example air and glass, through which radiation passes result in a discontinuous index of refraction, causing a certain amount of the impinging radiation to be reflected. By placing a coating of the appropriate thickness and index of refraction between the air and glass, it is possible to create destructive interference to eliminate undesirable reflection. Specifically, the use of a thickness equal to xc2xc of the wavelength at the center of the region for which anti-reflectance is desired results in a xc2xd wavelength phase difference for the reflected radiation. As a result, the radiation reflected at the coating-glass interface will be completely out of phase with the reflected wave at the air-coating interface. Both waves undergo a 180-degree phase change on reflection, and are out of phase because of the path difference. As a result, the reflected waves interfere destructively. Thus, if a lens is coated with a film having the right refractive index and a thickness of one-quarter wave, the reflection will be cancelled out and all of the incident radiation will enter (or leave) the lens.
Quarter-wave anti-reflecting coatings are commonly used in lens systems such as sunglasses, cameras, telescopes, automotive headlight systems, and binoculars. Both hard and soft lens coatings have been developed for use in these applications, varying in performance, cost, and lifetime. Anti-reflecting coatings utilized for these applications are generally designed to be effective over a single wave band. U.S. Pat. No. 5,894,366, which presents a means for improved automotive headlamp output, provides an example of a typical anti-reflecting coating comprising a substrate, a first layer, and a second layer, with the layers selected to optimize the passage of light in a particular wave-band from the headlamp and to give the light a particular tint.
Anti-reflection films have also been designed to be effective over closely neighboring portions of the ultraviolet and visible bands. U.S. Pat. No. 5,532,871, entitled xe2x80x9cTwo-Wavelength Antireflection Filmxe2x80x9d provides an example of an anti-reflection film capable of effective anti-reflection in two wavelength regions, for example, in an ultraviolet wavelength region of 200-300 nm and a visible wavelength region of 600-700 nm. It provides a substrate and a film consisting of alternately laminated layers of a low refractive index material and an intermediate refractive index material in order to form a light entrance side to the surface of the substrate. Another example of an anti-reflection film effective over two wave bands is provided in U.S. Pat. No. 4,997,241, entitled xe2x80x9cMulti-Layered Antireflection Film Preventing Reflection At Two Wavelength Regions.xe2x80x9d This reference discloses a multi-layered antireflection film preventing reflection in an ultraviolet wavelength region of 200-300 nm and a visible wavelength region of 600-700 nm.
Although two wave band anti-reflection coatings have been developed for the closely neighboring ultraviolet and visible wavelength regions, the present invention is concerned with developing a two band anti-reflection coating system effective in the infrared and millimeter-wave bands, which can be utilized in conjunction with a focal plane array responsive to both infrared and millimeter-waves.
In the past, systems designed for imaging in the infrared and millimeter-wave bands required two somewhat independent imaging and detection schemes. In general, reflecting optics used in these imaging systems tend to be large in size and cumbersome to use. U.S. Pat. No. 5,751,473 provides an example of a dual-waveband optical system for use in the infrared waveband, which incorporates a series of optical components to perform the desired imaging operation. With the increased need for smaller imaging devices such as space-based imaging and aircraft/spacecraft landing aids, a need has arisen for a lens system which provides imaging in both the infrared and millimeter-wave bands, yet which does not require a large and cumbersome multiple lens system. This is especially important in space-based applications where payload weight is cost-critical. In order to develop such a system, it is also important to develop a lens coating system that provides anti-reflection in the infrared and millimeter-wave bands.
It is therefore an object of the present invention to provide a lens coating arrangement that provides effective antireflection in both the infrared and millimeter-wave bands, which generally have a wavelength ratio on the order of 1:1000.
A dual-band millimeter-wave and infrared anti-reflecting coating is presented, including a first infrared anti-reflecting coating, with the first infrared anti-reflecting coating having an optical thickness of approximately xc2xc of the wavelength of a particular wavelength chosen from the infrared band; a millimeter-wave anti-reflecting coating disposed upon the first infrared anti-reflecting coating, with the millimeter-wave anti-reflecting coating having an optical thickness of approximately xc2xc of the wavelength of a particular wavelength chosen from the millimeter-wave band; and a second infrared anti-reflecting coating disposed upon the millimeter-wave anti-reflecting coating, with second infrared anti-reflecting coating having an optical thickness of approximately xc2xc of the wavelength of a particular wavelength chosen from the infrared band, wherein the particular wavelength chosen from the infrared band is the same for both the first infrared anti-reflecting coating and the second infrared anti-reflecting coating. The thickness of the particular materials chosen may be adjusted depending on the wavelengths for which a particular embodiment is designed. In actual use, the coating is typically applied to a substrate such as a lens. Typical lens materials include Zinc Sulfide (ZnS), Zinc Selenide (ZnSe), and Aluminum (III) Oxide (Al2O3), though any applicable material may be used.
In the special case of coherent infrared light incident on the lens, it is possible that the second infrared coating may be eliminated, and the millimeter-wave anti-reflecting coating is made to have one quarter wavelength thickness in the millimeter band and n/2+xc2xcxcex thickness in the infrared band where n represents the particular wavelength chosen from the infrared band.
These and other features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which like reference materials refer to like parts.